Hi,
I suppose that you mean not only the users, but also the addresses, the orders, etc.
In that case, it's not easy because there will be many orders, users, addresses, etc with the same ids on both websites.
The best will be to write a PHP script with MySQL queries to merge the data of the tables on the fly.
But that's not easy and will require a professional PHP developer to spend some time on it.
Merging the data manually in phpmyadmin with exports/imports of the tables is in the realm of possibilities but it won't be easy either and will take a lot of time. I don't really recommend it.